Tougher sanctions on Iran's oil and gas fields were proposed by Gordon Brown as part of international efforts to persuade Tehran to abandon its alleged attempts to acquire a nuclear bomb.
Energy companies would be banned from exploiting reserves for use by Iran if the sanctions outlined by the Prime Minister in a wide-ranging foreign affairs speech last night are endorsed by the EU or the UN. Those measures could be coupled with tougher economic sanctions by international banks if a report due shortly from the International Atomic Energy Authority (IAEA) shows that President Mahmoud Ahmadinejad's regime continues to defy the international community over the nuclear non-proliferation treaty.
Mapping out his strategy of "hard-headed internationalism" for Britain's future foreign policy, Mr Brown marked a shift from Tony Blair's readiness to act as foremost ally of the US and favoured confidant of President George W Bush. He signalled that in future Britain would work more closely with EU partners and through the UN. Mr Brown made it clear that Britain believes tougher sanctions – rather than the threats of military action – against Iran are starting to work, although his senior officials insisted that "nothing is ruled out".
Speaking at the Lord Mayor's Banquet in white tie and tails – an outfit he refused to wear when he was Chancellor – the Prime Minister also called for a standby civilian force including members of the police and judiciary to be created to deal with international crises such as Rwanda and Darfur.
Turning his attention to Pakistan, he reiterated the call for General Musharraf to step down as chief of the army and to press ahead with elections in January. A senior Downing Street source warned that it would be virtually impossible to hold free and fair elections with restrictions still in place. Action against Pakistan could include suspending its membership of the Commonwealth, which was discussed last night at a preparatory meeting of Commonwealth ministers. But any final decisions will be left to the full Commonwealth heads of government meeting later this month.
On Iran, Mr Brown answered defence chiefs who have warned that bellicose mutterings could prove counter-productive. He proposed internationally-agreed access to an enrichment bond or nuclear fuel bank to help non-nuclear states such as Iran to acquire the new sources of energy they need.
His plan closely follows the proposal put forward during the controversial state visit to Britain by King Abdullah of Saudi Arabia, who suggested Switzerland, as a neutral country, could hold the fuel bank. His officials said this offered an incentive to the Iranian leadership to abandon their weapons-grade nuclear enrichment programme.
A fuel bank would avoid Iran having to process nuclear material and would make it more difficult for the Iranian President to argue that his country's enrichment process was for peaceful nuclear power.
Mr Brown said: "This offer should be made only as long as these countries renounce nuclear weapons and meet internationally-enforced non-proliferation standards.
"Iran has a choice – confrontation with the international community leading to a tightening of sanctions or, if it changes its approach and ends support for terrorism, a transformed relationship with the world."
The IAEA will report shortly to the UN and officials said that, if it proved Iran was continuing to defy the international community, Britain would push for tougher sanctions both in the UN and by the EU. It could mean that with Russia backing Iran, the sanctions would have to be imposed through the EU alone.
However, the core of Mr Brown's speech addressed controversial remarks by the foreign minister, Lord Malloch-Brown, that Mr Brown and Mr Bush would "no longer be joined at the hip".
Without mentioning Lord Malloch-Brown by name, Mr Brown said he would have "no truck with anti-Americanism" and stressed that the US remained Britain's "most important bilateral relationship".
But he said Britain had to be guided by "hard-headed internationalism – internationalist because global challenges need global solutions and nations must co-operate across borders, often with hard-headed intervention ... hard-headed because we will not shirk from the difficult long-term decisions."
He added: "Today, a nation's self-interest will be found not in isolation but in co-operation to overcome shared challenges – we must bring to life these shared interests and shared values by practical proposals to create the architecture."
The major challenges threatening destabilisation included terrorism, rogue states, pandemics, and mass migration, he said.
Mr Brown made only a passing reference to the conflict in Afghanistan, in spite of reports that he is planning an initiative to pay farmers to give up cultivating poppies.